1. Field of the Invention
This invention relates to power converters and more specifically to a packaging architecture that provides for vertical power flow that is effective for providing lower supply voltages, dynamic voltage scaling, multiple supply voltages, and fast transient response and tight regulation.
2. Description of the Related Art
Power converters are key components in many military and commercial systems and they often govern size and performance. Power density, efficiency and reliability are key characteristics used to evaluate the characteristics of power converters. Transformers and inductors used within these power converters may be large and bulky and often limit their efficiency, power density and reliability. These deficiencies can be improved by using a high-frequency “switch-mode” architecture instead of a traditional low frequency transformer and by replacing conventional core-and-wire designs with “planar magnetics”. Planar magnetics offer several advantages, especially for low-power dc—dc converter applications, such as low converter profile, improved power density and reliability, reduced cost due to the elimination of discrete magnetic components, and close coupling between different windings.
As shown in FIG. 1, a conventional switch-mode power converter 10 for transforming an input voltage Vin, e.g. 48V, to one or more ultra low supply voltages VS, e.g., 1.5V, 3.3V, to drive a load 12 utilizes horizontal packaging in which components are mounted on the same multi-layer printed circuit board (PCB) 14, and power flows sequentially over long interconnects from input to the output side. Moreover, multiple secondary windings and cross regulation are utilized for the generation and control of multiple supply voltages. Components such as an input filter 16, primary switches 18, a primary control IC 20, a transformer 22, secondary devices 24, a secondary control IC 26, an output inductor 28 and an output capacitor 30 are mounted on the board 14, forming what will be referred to hereafter as a “horizontal package”. Arrow 32 indicates that the power flows through the different components in the horizontal direction from the input to the output and is coupled horizontally via traces on the PCB to load. Other internal layers may be used for interconnections, ground planes, or some active or passive devices in MCM-type (multi-chip module) or embedded packaging.
A popular implementation of the dc/dc switch-mode converter 10 to supply a single regulated output voltage incorporates a drive circuit 34 having a double-ended, half-bridge topology and a current-doubler rectifier (CDR) circuit 36 shown in FIG. 2 (U.S. Pat. No. 6,549,436 issued Apr. 15, 2003). Early CDR circuits used three separate magnetic components, namely, one transformer and two inductors. The illustrated CDR is based on an integrated magnetic implementation in which the transformer and inductors are combined into a single magnetic structure with one magnetic core. The integrated magnetic implementation is further refined to include an output inductor that increases the effective filtering inductance (See U.S. Pat. No. 6,549,436).
The drive circuit 34 comprises first and second input filter capacitors 40 and 42 and first and second primary switches 44 and 46, e.g. power MOSFETs. The capacitors 40 and 42 and switches 44 and 46 process power from a dc voltage source Vin at input terminals 48 and 50. The drive circuit 34 provides a pulse width modulated voltage to the CDR's split-primary winding arrangement 52 and provides an ac voltage at the input terminals of the integrated magnetics.
The CDR circuit 36 comprises a magnetic core 54, the split-primary winding arrangement 52, a secondary winding arrangement 56, an output capacitor 58, and first and second secondary switches 59 and 60, and first and second rectifiers 61 and 62 connected in parallel across the respective switches. The switches 59 and 60 function as diodes, termed synchronous rectification, and can be replaced by diodes only. The magnetic core 54 comprises a center leg 64 and a first outer leg 66 and a second outer leg 68 disposed on opposite sides of the center leg 64. A plate 67 on the outer legs forms an air gap 69 with the center leg to prevent saturation of the core.
The split-primary winding arrangement 52 comprises a primary winding 70 that is wound around the outer leg 66 and a second primary winding 72 that is wound around the outer leg 68. The secondary winding arrangement 56 comprises first, second and third secondary windings 74, 76, 78 that are wound around legs 66, 68 and 64, respectively. The outer leg windings 74 and 76 provide both the secondary windings for the transformer and the output inductors. The center leg inductor winding 78 increases the filter inductance of the CDR circuit thereby reducing the voltage and current ripple and improving efficiency.
The inductor winding 78 is connected in series with the output capacitor 58. The output capacitor has first and second terminals 80 and 82, which form the output terminals of the integrated current-doubler rectifier 36 and the dc/dc converter circuit 10 shown in FIG. 1 for connection to the load. The secondary switch 59 and rectifier 61 are connected in parallel between the output capacitor 58 and the winding 74. The secondary switch 60 and rectifier 62 are connected in parallel between the output capacitor 58 and the winding 76.
In operation, a dc voltage is applied to the capacitors 40 and 42 and the primary switches 44 and 46 via input terminals 48 and 50. A primary control IC 84 controls the primary switches such that at most only one switch is on at a time and synthesizes a high frequency AC voltage that is applied to the primary windings 70 and 72. This causes a current to flow in the secondary windings 74, 76 and 78. A current i1 flows in the switch-diode pair 59–61, a current i2 flows in the switch-diode pair 60–62, and a current i3 to flow in the secondary winding 78 (where i1+i2=i3), though ordinarily not all at the same time. One of the currents i1 or i2 is zero during power transfer periods, while in the free wheeling periods the load current to node 80 is shared among them. A secondary control IC 86 controls secondary switches 59 and 60 so that current i1 flowing through winding 74 is rectified by the switch-diode pair 59–61 and the current i2 flowing through the winding 76 is rectified by the switch-diode pair 60–62. Current i3 charges the output capacitor 58 to produce a DC output voltage across output nodes 80 and 82 so that a regulated power is delivered to the load. Power flows from the input terminals horizontally through the primary switches, the transformer plus inductors, secondary switches to the output terminals for connection to a load on the same board.
As shown in FIGS. 3a and 3b, the primary and secondary winding arrangements are implemented with a multi-layer printed circuit (PCB) 90 having copper traces that form the various horizontal windings in the plane of the PCB. E-core 54 is positioned underneath the PCB so that its outer legs 66 and 68 extend through holes in the PCB that coincide with the edges of primary and secondary windings 70 and 74 and 72 and 76, respectively, and its center leg 64 extends through a hole that allows inductor winding 78 to be wound around it. Required creepage distance is maintained between the windings and the core during fabrication. Plate 67 rests on the outer legs forming an air gap 69 with the center leg. Vias 92 in the PCB are used to connect the primary windings in series to form a multi-turn primary and to connect the secondary windings in parallel to form a single-turn secondary with reduced resistance. The windings are terminated in the plane of the PCB so that power flows horizontally from the primary side to the secondary side.
Among the various power reduction and power management requirements for developing systems, the needs for lower supply voltages, dynamic voltage scaling, multiple supply voltages, and fast transient response with tight regulation will have the most dramatic effects on power converter design. While each individual requirement represents a challenge for the power converter design and packaging, it is the combination of them all together that is pushing the existing power conversion technology to its limit.
The conventional horizontal package has fundamental limitations that will render it ineffective for these developing applications, including a) inherently low efficiency, especially at sub-1V output, due to the long internal interconnects and the associated high conduction losses, b) a difficult 1-D interface with the load, c) inability to supply tightly regulated multiple outputs, and (d) switching frequency limitation due to the inductive and capacitive parasitics inherent in long interconnects. The needs for coordination among multiple supply voltages, such as sequencing, also makes it difficult to use multiple, individually controlled single-output converters. In addition, conventional control design focuses on constant output regulation with steady-state load, which cannot meet the future needs for dynamic voltage scaling and fast transient responses.